Random flights in confining interfacial systems

Abstract

Porous materials, concentrated colloidal suspensions are examples of confining systemsdeveloping large specific surface and presenting a rich variety of shapes. Such an interfacialconfinement strongly influences the molecular dynamics of embedded fluids and thediffusive motion of entrapped Brownian particles. An individual trajectory near theinterface can be described as an alternate succession of adsorption steps andrandom flights in the bulk. Statistical properties of these random flights in variousinterfacial confining systems are needed as prerequisites in order to understand thefull transport process. Related to first passage processes, these properties play acentral role in numerous problems such as the mean first exit time in a boundeddomain, heterogeneous catalytic reactivity and nuclear magnetic relaxation incomplex and biological fluids. In the present work, we first consider the variouspossibilities of connecting two points of a smooth interface by a random flight in thebulk. Second, we analyse from the theoretical and experimental points of view away to probe Brownian flight statistics. From the experimental point of view, weinvestigate the slow fluid dynamics near some colloidal interfaces by field-cycling NMRrelaxometry. This is a way to follow slow dynamical correlations from 1 ns to10 µs. This spectroscopy appears to be a good choice, considering that the algebraic nature ofthe probability of the first return to a surface builds a long-time memory. The experimentalpart confirms that the embedded fluid dynamics is sensitive to possible morphologiccrossover and provides information about interface geometry. We also believe that such anapproach can be used to probe interfacial dynamics by itself, for example in the case ofa colloidal system undergoing a phase transition (dynamical arrest, rotationalblockage,...).